What is momentum?
What do you think you have more chance of surviving?
A bus hitting you at 20mph or a bike hitting you at 20mph?
A bullet being thrown at you or a bullet being fired at you?
You've probably chosen the bike hitting you and a bullet being thrown at you. The reasons that these are less likely to hurt you are because of momentum. The momentum of an object is made up of its mass and velocity. The equation for momentum is...
As the bike has less mass than the bus it has a lower momentum even though they are both travelling at the same speed. A bullet being thrown has less velocity than the bullet being fired even though they have the same mass and so has less momentum.
Momentum in an explosion
Whenever an explosion happens, the momentum is shared equally between the two objects. How each object moves depends on the mass and velocity. For example - when a bullet is fired in a gun, half of the momentum goes into the bullet, which as it has a small mass travels out of the gun quickly (with high velocity), the other half of the momentum goes into the gun, which, as it has a far bigger mass, causes a much smaller movement in the gun - this causes the gun to 'kick'. Watch the videos below to see examples of this...
In the examples of guns firing, the momentum on the bullet will be positive, but the momentum on the gun will be negative. This is because the gun and bullet travel in opposite directions so one has to have a positive velocity while the other has a negative velocity.
Momentum in Motion
Momentum is always conserved between two objects - for example - if a car travelling at 5m/s hits a stationary car (with the brakes off) then both cars will move off. The total momentum will be the same after the cars hit as it was before as the diagram below shows...
Notice in the above diagram how the momentum is 1000kg m/s on before and after the collision.
Elastic and Inelastic Collisions
Collisions can be described as elastic or inelastic
An elastic collision is where the objects collide and move off at different speeds (and possibly in different directions), for example a set of snooker balls which all move off in different directions when hit by the white ball...
An inelastic collision is where the two objects hit and then move off in the same direction at the same speed as if they were one object...